1. Field of the Invention
This invention relates to wire electrode type electric discharge machining apparatus, and more particularly to an improvement of the machining accuracy thereof.
2. Description of the Prior Art
FIG. 1 is a schematic diagram outlining the arrangement of a conventional wire electrode type electric discharge machining apparatus. Such an apparatus is described in Published Unexamined Japanese patent Application No. 52129/1981.
In FIG. 1, reference numeral 1 designates a wire electrode; 2, a workpiece to be machined; 3, an X-slider for moving the workpiece 2 right and left in FIG. 2; 4, a Y-slider for moving the workpiece in parallel with the surface of the drawing; 5, a servo motor for driving the X-slider 3; 6, a servo motor for driving the Y-slider 4; 7, a servo amplifier for supplying current to the servo motor 5; 8, a servo amplifier for supplying current to the servo motor 6; 9, a machining power source for applying a pulse voltage between the wire electrode 1 and the workpiece 2; 10, a detector for detecting an average machining voltage between the wire electrode 1 and the workpiece; and 11, a control unit for controlling the servo amplifiers 7 and 8 according to the output signal of the detector 10 and a predetermined machining program.
The operation of the electric discharge machine thus organized will be described.
The wire electrode 1 is fed at a predetermined speed, while the machining power source 9 applies the pulse voltage between the wire electrode 1 and the workpiece 2 to cause electric discharge therebetween to machine the workpiece 2. In this operation, the control unit 11 applies movement instruction signals to the servo amplifiers 7 and 8, respectively, according to the machining program, and in response to these signals, the servo motors 5 and 6 drive the X-slider 3 and the Y-slider 4, respectively, so that the workpiece is machined as required.
In general, machining conditions change frequently. Therefore, in response to the average voltage between the electrode detected by the detector 10, the control unit, drives the X-slider 3 and the Y-slider 4 at suitable feed speeds so that the machining gap between the wire electrode 1 and the workpiece 2 is maintained constant.
In a machining operation, generally after a coarse machining operation, an end-face finish-machining operation is carried out several times so that the resultant configuration and surface roughness are satisfactory in accuracy. The configuration accuracy of a finish-machined workpiece depends on the interelectrode gap, and therefore, in the case where it is required to machine a workpiece with high accuracy, it is essential to maintain the interelectrode gap at a constant. FIG. 2 is an enlarged diagram showing the wire electrode 1 and the workpiece 2 in the electric discharge machining operation. In a conventional ordinary system in which the average voltage is controlled so as to be constant, as the amount of removal L increases as the machining speed U is decreased, as a result of which in the interelectrode area (D in the figure) the machining integration effect is increased, and the interelectrode gap Gs is thus greater than 60. That is, if the amount of removal L changes while the machining electrical conditions and the average servo voltage are maintained unchanged, then the interelectrode gap becomes nonuniform, and therefore the workpiece machined workpiece is low in configuration accuracy. FIG. 3 is a graphical representation indicating amounts of removal L with interelectrode gap Gs with the machining electrical conditions and the average servo voltage maintained unchanged. As is apparent from FIG. 3, the interelectrode gap Gs changes greatly with variation in the amount of removal L.
In an actual workpiece machining operation, the amount of removal L is maximum at a corner. FIGS. 4(a)n and 4(b) are enlarged views showing the wire electrode 1 and the workpiece 2 in an inside-corner finish-machining operation. As is apparent from FIGS. 4(a) and 4(b), the amounts of removal L (L.sub.2 through L.sub.4) at the corner are much larger than those (L.sub.0 and L.sub.5) in a straight machining operation. FIG. 5 shows the variation in the amount of removal L at an inside corner. As is clear from FIG. 5, the amount of removal L decreases starting at a position before the start point of the corner (cf. H1 in FIG. 5) until it reaches a certain value, the amount of removal thus increased is maintained at the certain value for a while, and then the amount of removal decreases starting at a position before the end point of the corner (cf. H3 in FIG. 5) until it reaches the value in the straight machining operation again.
Thus, especially at the inside corner, the amount of removal L increases and the interelectrode gap Gs also increases, with the result that the machined workpiece is considerably low in configuration accuracy due to the amount of excessive cut d as shown in FIG. 6. On the other hand, at the outside corner, the amount of removal L decreases and the interelectrode gap Gs also decreases, as a result of which the machined workpiece is also considerably low in configuration accuracy.
The conventional wire electrode type electric discharge machining apparatus thus constructed suffers from a difficulty in that especially at a corner the interelectrode gap changes with the amount of removal, as a result of which the machined workpiece is considerably low in configuration accuracy.